like what?
also it's weird argument "constantly inventing new js apis"
so what? their usefulnesss should be determined in their own context, not by the fact that they're a js api; unless you're a child who wages weird programming language culture wars on the internet and thinks anything that isn't the language they use is inferior
If Microsoft isn't able to maintain their own browser, the idea that some other org out there is going to have the resources to fork it is laughable.
You are essentially arguing that a monopoly position in this particular industry minimizes what is known as "welfare loss". That would be quite an exceptional claim, IMHO
This in general only happens within market areas that appeal to the formation of so-called "natural monopolies" - typical examples being water, postal services, electricity, and the like (utilities). The rationale being that the monopoly formation in this case spares society as a whole for excessive costs as these industries are characterized by a very high proportion of fixed costs (infra, machinery, ect) relative to variable costs.
With browsers it seems that on the contrary the overwhelming majority of development costs are indeed variable (man hours) which -- all else equal -- would support the assumption that competition in this market would be better for society as whole, and that a monopoly here would be a loss for society (and eventually it would fall).